1. Field of the Invention
The present invention relates to an USB device which is connected to devices through a USB interface and is capable of feeding current to those devices.
2. Background Art
The USB interface is a 4-wire serial interface, and contains two data lines (D+, D−), a power supplying line (Vbus), and a ground line (GND). The USB device is capable of supplying electric power to the devices through power supplying lines. Therefore, the devices are operable in a state that power sources are not contained in the devices. The USB interface standards describe that the maximum electric power allowed to be supplied is 5 V and 500 mA.
Most of the host side USB devices each having the current supplying capability are provided with an overcurrent protecting device which operates when a current value of current fed to the device exceeds 500 mA as a normal value, and restricts the mount of current to flow. The overcurrent protecting device prevents the related parts of the device from being damaged by the overcurrent.
The conventional USB device restricts the supplied current when it reaches 500 mA, but cannot detect an abnormality of such overcurrent. When a trouble occurs in the power source circuit in the device usually consuming the current of only 100 mA, for example, overcurrent flows. In this case, the host side USB device merely limits the current to below 500 mA, and cannot detect such an abnormal state that current of 500 mA or larger flows through the device consuming the current of only 100 mA.
The host side USB device, widely used, includes a plurality of USB ports at which the USB device maybe coupled to a plurality of devices. Such a USB device has also the capability to supply electric power to the devices through the USB ports. In this case, a single power source is shared with those USB ports, and delivers electric power to the USB ports.
In the system containing the USB device and the plurality of devices connected to the USB device through the USB ports, when a trouble occurs in the device coupled to a certain USB port, and the amount of current increases, the voltage drop occurs and will adversely affect other devices.